Hi,
You could give this a try:
[list=][*]extract the B&LE's texturing.bin from the .ap on your local machine
[*]open it and you own route's texturing.bin with RW Tools and copy/paste whatever texture entries you want to use on your own route over into your own texturing.bin in your route's .bin
[*]delete the extracted B&LE .bin from step 1 again
[*]see if it works

[/list]
I don't actually know if this will work or not, but in theory it should. All the texturing.bin does is list file/path references to the terrain texture graphics and give them names and an order which you can use in the editor. Whether or not the linked files are inside an .ap file should (again, in theory) not matter. If it works, there would be no reason to give your users complicated "extract this and that" instructions as all they'd need would be your own texturing.bin with the links to the B&LE graphics inside.
Cheers
Michael
PS: Please note that when editing the texturing.bin, it is best to only add new entries at the end of the list. Changing the order of the textures will mess up any textures you have already painted on your route, as the MixMap seems to identify the terrain textures by its order from 1st to last in the texturing.bin.